The invention relates to systems for characterizing optical components, and more particularly to systems for characterizing optical components using interferometer-based optical network analysis.
Characterizing two ports of an optical component involves inputting an optical signal into one of the ports and measuring the optical response signals that exit the two ports of the optical component. In a two port configuration, one optical response signal results from reflection by the optical component and the other optical response signal results from transmission through the optical component. FIG. 1 depicts a basic block diagram of a test system 100 for characterizing a two port optical component 102, where the optical component is commonly referred to as the component under test or the device under test (DUT). The test system includes an optical signal source 104, a coupler 106, and two optical component analyzers (OCAs) 108 and 110. The optical signal source is optically connected to one port (i.e., the input port) 112 of the optical component so that an input signal can be applied to the optical component. One of the OCAs is optically connected to the input port of the optical component via the coupler. The coupler allows the OCA 108 to receive the optical response signal that results from reflection of the input signal by the optical component. The other OCA 110 is optically connected to the output port 114 of the optical component to receive the optical response signal that results from transmission of the input signal through the optical component. Because the test system includes two OCAs, both ports of the optical component can be simultaneously characterized.
In the above-described test system, the optical signal source provides an input optical signal (also referred to a stimulus) to the optical component and the optical response signals that result from the input optical signal are directly measured by the OCAs. That is, the optical response signals are not combined or mixed with any other optical signals before being detected by the OCAs. Although this direct measurement technique for characterizing an optical component works well for measuring the scalar quantities, such as bandwidth, insertion loss, and gain or loss of the component under test, the resolution and bandwidth range that can be achieved by direct measurement of the optical response signals is limited. In addition, the direct measurement technique cannot be used to characterize the dispersion properties of the component under test. Measuring the dispersion properties of the component under test in this case requires the use of additional specialized equipment.
The desire to multiplex more, and therefore narrower, channels into a single optical fiber to achieve cost effective data transfer at very high data rates has driven the need for higher resolution optical network analysis techniques that are able to efficiently characterize the dispersion properties of an optical component. One high resolution optical spectrum analysis technique, known as interferometer-based optical spectrum analysis, involves combining two optical signals and measuring the interference signal that results from combining the two signals. Test systems that utilize interferometric optical spectrum analysis to characterize an optical component are known. However, these test systems only allow the characterization of one port of a component under test at a time and cannot characterize the dispersion properties of the component under test. For example, known test systems can either characterize reflection at the input port of a component under test or transmission at the output port of the component under test. To characterize the other port of the component under test, the optical component must be disconnected from the test system, reoriented, and then reconnected to the test system. Although each port of an optical component can be tested serially by adjusting the orientation of the optical component, it is desirable to be able to simultaneously characterize at least two ports of an optical component using interferometer-based optical spectrum analysis without having to adjust the orientation of the component under test.
In view of the limitations of known systems for characterizing optical components, what is needed is a system for characterizing an optical component that allows simultaneous interferometric analysis of a component under test in reflection and transmission.
A test structure that supports simultaneous characterization of a two port optical component optically connects an optical local oscillator source, receivers, and a signal processor to the optical component that is to be tested, also referred to as the component under test or the DUT. The test structure includes an input port for receiving an input signal from the optical local oscillator source, two test ports for connecting the test structure to a component under test, separate optical paths for receiving reflected and transmitted optical response signals from the component under test, and optical components for combining a first portion of the input signal with the reflected optical response signal and for combining a second portion of the input signal with the transmitted optical response signal. The local oscillator source provides the input signal to the component under test, the receivers convert the combined optical signals into electrical signals, and the signal processor processes the electrical signals to generate output signals that are indicative of an optical characteristic of the component under test. Because the optical response signals are combined with portions of the input signal before being converted to electrical signals, interferometer-based optical network analysis (also known as swept homodyne analysis) can be used to obtain high resolution optical characterization of the component under test. The swept homodyne technique allows the characterization of both loss and dispersion properties in the relevant wavelength range of the component under test.
In an embodiment of the test structure, optical couplers connected by optical fibers are utilized to combine the input signal and optical response signals and in another embodiment of the test structure, the optical couplers and optical paths are integrated into a single substrate.
A switch can be added between the input port and the two test ports to enable the component under test to be tested in two directions without having to reverse the orientation of the component under test relative to the test structure. In an embodiment where the optical couplers are connected by optical fibers, a 1xc3x972 switch is integrated into the test structure. In a single substrate test structure, the switch is externally connected to intermediate switch ports.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.